Branching ratios for competing channels in the reaction of HOCO radicals with NO

The yields of CO2 from the reactions of HOCO radicals with O-2 and with NO have been compared using the technique of time-resolved infrared absorption . In two separate series of experiments, HOCO radicals have been generated by (i) broadband ultraviolet photolysis of acrylic acid (CH2CHCOOH) and (ii ) the reaction, Cl+HCOOH --> HCl + HOCO, following pulsed laser photolysis of Cl-2 at 351 nm. In the latter experiments, the observed CO2 yields were corrected for the loss of Cl atoms by the reactions Cl+NO+He --> ClNO+He an d Cl + ClNO --> Cl-2 + NO. Our measurements on the rate of Cl+NO+He yield a value of the third-order rate constant of (5.1 +/- 0.7) x 10(-32) cm(6) mo lecule(-2) s(-1). The results of the main experiments show that the yield o f CO2 from the reaction between NO+HOCO is appreciably less than that from O-2 + HOCO and is dependent on the total pressure in the range 10 to 100 To rr. We interpret this as showing that the reactions and HOCO + NO --> HNO CO2 and HOCO+NO(+M) --> HOC(O)NO(+M) occur at similar rates, with the latt er reaction being favoured at the higher total pressure.